CVE-2026-45866

In the Linux kernel, the following vulnerability has been resolved:

serial: caif: fix use-after-free in caif_serial ldisc_close()

There is a use-after-free bug in caif_serial where handle_tx() may

access ser->tty after the tty has been freed.

The race condition occurs between ldisc_close() and packet transmission:

CPU 0 (close) CPU 1 (xmit)

------------- ------------

ldisc_close()

tty_kref_put(ser->tty)

[tty may be freed here]

<-- race window -->

caif_xmit()

handle_tx()

tty = ser->tty // dangling ptr

tty->ops->write() // UAF!

schedule_work()

ser_release()

unregister_netdevice()

The root cause is that tty_kref_put() is called in ldisc_close() while

the network device is still active and can receive packets.

Since ser and tty have a 1:1 binding relationship with consistent

lifecycles (ser is allocated in ldisc_open and freed in ser_release

via unregister_netdevice, and each ser binds exactly one tty), we can

safely defer the tty reference release to ser_release() where the

network device is unregistered.

Fix this by moving tty_kref_put() from ldisc_close() to ser_release(),

after unregister_netdevice(). This ensures the tty reference is held

as long as the network device exists, preventing the UAF.

Note: We save ser->tty before unregister_netdevice() because ser is

embedded in netdev's private data and will be freed along with netdev

(needs_free_netdev = true).

How to reproduce: Add mdelay(500) at the beginning of ldisc_close()

to widen the race window, then run the reproducer program [1].

Note: There is a separate deadloop issue in handle_tx() when using

PORT_UNKNOWN serial ports (e.g., /dev/ttyS3 in QEMU without proper

serial backend). This deadloop exists even without this patch,

and is likely caused by inconsistency between uart_write_room() and

uart_write() in serial core. It has been addressed in a separate

patch [2].

KASAN report:

==================================================================

BUG: KASAN: slab-use-after-free in handle_tx+0x5d1/0x620

Read of size 1 at addr ffff8881131e1490 by task caif_uaf_trigge/9929

Call Trace:

<TASK>

dump_stack_lvl+0x10e/0x1f0

print_report+0xd0/0x630

kasan_report+0xe4/0x120

handle_tx+0x5d1/0x620

dev_hard_start_xmit+0x9d/0x6c0

__dev_queue_xmit+0x6e2/0x4410

packet_xmit+0x243/0x360

packet_sendmsg+0x26cf/0x5500

__sys_sendto+0x4a3/0x520

__x64_sys_sendto+0xe0/0x1c0

do_syscall_64+0xc9/0xf80

entry_SYSCALL_64_after_hwframe+0x77/0x7f

RIP: 0033:0x7f615df2c0d7

Allocated by task 9930:

Freed by task 64:

Last potentially related work creation:

The buggy address belongs to the object at ffff8881131e1000

which belongs to the cache kmalloc-cg-2k of size 2048

The buggy address is located 1168 bytes inside of

freed 2048-byte region [ffff8881131e1000, ffff8881131e1800)

The buggy address belongs to the physical page:

page_owner tracks the page as allocated

page last free pid 9778 tgid 9778 stack trace:

Memory state around the buggy address:

ffff8881131e1380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

ffff8881131e1400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

>ffff8881131e1480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

^

ffff8881131e1500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

ffff8881131e1580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

==================================================================

[1]: https://gist.github.com/mrpre/f683f244544f7b11e7fa87df9e6c2eeb

[2]: https://lore.kernel.org/linux-serial/[email protected]/T/#u